The present invention relates to the field of arrangements relating to reflector antennas, and in particular to that part of this field concerned with reflector antennas that include subreflectors.
Many technical applications include some form of antenna function for sending or receiving radio signals. Examples of such applications are radio apparatus, TV apparatus, mobile telephony systems, radio communication links and radar systems.
The requirement placed on the directional effect of an antenna varies in accordance with the application concerned. A radio apparatus shall be capable of receiving signals from different radio stations, regardless of where the apparatus is located, and the antenna should therefore be equally as receptive in all directions in a horizontal plane. On the other hand, a TV receiver shall only be receptive to signals arriving from the nearest TV mast or from a TV satellite. Thus, the antenna of a TV receiver should be positioned so that it is particularly receptive to signals arriving from a certain direction, and signals that arrive from other directions shall be suppressed to the greatest possible extent. This also applies, for instance, to antennas for radio links. Radar apparatus shall normally both transmit and receive in a certain direction, which shall also be capable of being changed so that the radar can receive omnidirectional information relating to the surroundings. It is also desirable in respect of radar apparatus that the antenna will function to suppress signals from directions other than the direction in which the radar currently transmits and receives at that moment in time.
A common type of directional antenna is the so-called reflector antenna. A reflector antenna will normally include a main reflector and a feed. The feed is placed in front of the main reflector and is adapted to transmit or receive electromagnetic radiation reflected onto the main reflector. A common type of feed includes a waveguide or corresponding device, and a subreflector. In the transmission of electromagnetic radiation, the waveguide is excited to deliver electromagnetic radiation of a predetermined kind. The radiation emitted from the waveguide is first reflected against the subreflector and then against the main reflector. Electromagnetic signals can also be received by the reflector antenna. In the case of reception, the beam path will, of course, travel in the reverse direction to that travelled in the case of transmission. The dimensions of the main reflector are conveniently much larger than the wavelength of the signals used in the application in question. The main reflector is formed to combine signals that are transmitted (or are incoming) in a certain direction, in a manner that is suitable in the context concerned. The directional sensitivity, or receptiveness, of the reflector antenna can be changed by realigning the antenna mechanically.
Technical specifications have been compiled in order to characterise the quality of the directional properties of reflector antennas. For instance, ETSI (European Telecommunications Standard Institute) have produced a specificationxe2x80x94ETS 300 833xe2x80x94that specifies radio link antenna requirements. The specification states requirements concerning the radiation diagram of the reflector antenna in a horizontal plane. For a number of frequency ranges, the specification states, inter alia, the requirement regarding side lobe levels (both with regard to co-polarisation and cross-polarisation). Several numbered classes are specified for each frequency range and the greater the number, the stricter the requirements placed on the suppression of side lobes.
In order to better utilise the radio frequency spectrum, it is usual to adapt radio link antennas for the use of either horizontal polarisation (horizontal E-field) or vertical polarisation (vertical E-field). It is, of course, beneficial if one and the same reflector antenna can be used for both horizontal polarisation and vertical polarisation, for instance by rotating the feed and the subreflector. In order to make this possible, it is therefore necessary for the reflector antenna to be adapted to enable the quality requirements placed on the radiation diagram (for instance, in accordance with the above-mentioned ETSI specification) to be achieved both in an E-plane in respect of horizontal polarisation and in an H-plane in respect of vertical polarisation.
WO, A1, 87/07771 teaches a reflector antenna comprising a feedxe2x80x94a so-called hat feedxe2x80x94that includes a subreflector. It would appear that the subreflector is constructed, inter alia, to achieve with the reflector antenna a radiation diagram which in an H-plane coincides with a radiation diagram in an E-plane to the greatest possible extent. The subreflector is rotationally symmetrical about a centre axis and includes a centrally positioned conical spreader which is intended to be placed in front of the aperture of a waveguide in the feed. That part of the reflective structure of the subreflector located outside the spreader is essentially planar, although it includes circular corrugations (grooves) of a constant depth correspond approximately to one-quarter wavelength. The construction of the subreflector also enables the hat feed to be made very compact.
Reflector antennas that include hat feeds function very efficiently in general. However, reflector antennas that include a hat feed do not function satisfactorily in some cases. For instance, when measuring at high frequencies on a 0.3 m reflector antenna that included a hat feed, it was found that the reflector antenna was unable to meet the requirements of ETSI class 3 (30-47 GHz) in the H-plane. The radiation exceeded specified levels in the region nearest the main lobe and for angles around 60xc2x0 in relation to the main lobe (so-called spillover lobes, in other words direct radiation from the feed that failed to impinge on the main reflector). The reflector antenna equipped with the hat feed essentially met the requirements of ETSI class 3 in respect of the E-plane.
One drawback with the hat feed is thus that it is unable to achieve a radiation diagram with high suppression of side lobes in both the H-plane and the E-plane under all circumstances.
The present invention addresses chiefly the problem of obtaining an improved subreflector which, when used in a reflector antenna, enables the reflector antenna to obtain a radiation diagram with high suppression of side lobes in both the H-plane and the E-plane.
In brief, the problem addressed above is solved by providing the subreflector with an improved reflective structure.
Accordingly, one object of the invention is to provide a subreflector that is an improvement with respect to achieving radiation diagrams of predetermined quality in different planes, wherein the invention also includes a feed that includes one such subreflector and also a reflector antenna that includes such a subreflector.
More specifically, the above addressed problem is solved as follows: The reflective structure of the subreflector includes at least two different geometries which have been designed specifically to obtain radiation diagrams that have effective suppression of side lobes in both the E-plane and the H-plane.
An essential advantage afforded by the invention is that it enables the procurement of reflector antennas that can be used for both horizontal polarisation and vertical polarisation in applications where high quality is required of the reflector antenna radiation diagram in a horizontal plane (or in a vertical plane).
The invention will now be described in more detail with reference to preferred exemplifying embodiments thereof and also with reference to the accompanying drawings.